Among axial-gap type PM motors, one that is shown in FIG. 13 has been well known.
As shown in FIG. 13, this motor is so structured as to be arranged in a housing (frame) 1, and comprises a rotating shaft 7 rotatably supported by bearings 2, 3 fixed to the housing 1, a rotor 4 integrally attached to the rotating shaft 7, and a stator 5 arranged facing the rotor 4 and fixed to the housing 1.
The rotor 4 is made of a circular disc 6 of a ferromagnetic material, such as iron, and the rotating shaft 7 is mounted at the center of the disc 6 and rotatably supported at both ends thereof by the bearings 2, 3. On the surface of the circular disc 6 which faces the stator 5, N-pole magnets 8 and S-pole magnets 9 are arranged alternately in the circumferential direction as shown in FIG. 14.
The stator 5 comprises an annular stator yoke 10 fixed to the housing 1, and a plurality of teeth 11, which are press-fitted in a plurality of holes formed in the circumferential direction of the stator yoke 10. The stator teeth 11 are wound with coils 12 of wire.
Meanwhile, in electric cars, an in-wheel motor is mounted in the wheel to realize independent drive of the wheel. The in-wheel motor, which includes a reduction gear (change gearbox), is required to be flatter and thinner in structure. Flat-type PM motors such as the one mentioned above will find application as an in-wheel motor with a reduction gear.
In conventional axial-gap PM motors mentioned above, however, because they are incapable of utilizing reluctance torque, when they are used for in-wheel motors in electric-powered cars, there is a problem that the available range of rotation speed N is narrow.
Incidentally, the above-mentioned axial-gap type PM motor may be used as a generator.
Therefore, when used in a motorcycle or the like, in addition to the use as a starter motor to start the engine, and after the engine has started, this motor may be switched to a generator mode by using the driving force of the engine. This motor is hereafter referred to a starter-motor-cum-generator.
In other words, the starter-motor-cum-generator is used as a starter motor to start the engine and after the engine has started, it is used as a generator.
When the axial-gap type motor is used as a starter-motor-cum-generator as described above, the motor needs to generate a high torque to start the engine. A general method to realize a high torque has been to increase the magnetic force of the magnet of the motor.
With a conventional axial-gap type motor, since there is little hope that this motor gives reluctance torque, there has been no other choice but to increase the magnetic force of the magnet.
However, when the axial-gap motor is used as the starter-cum-generator, after the engine has started, the motor switches its operation to a generator mode, and need not generate a high torque. On the contrary, if the magnetic force of the motor magnet is increased to generate a high torque, when the generator is driven at high speed to generate electric power, the battery is likely to be overcharged, so that it is necessary to provide a voltage reducing circuit to protect batteries.
If the starter-motor-cum-generator is to be formed by using an axial-gap motor, the magnetic force of the motor magnet is desired to be large to secure a high torque at low rotation speed (when starting a car). However, to suppress overcharging of the battery at high rotation speed, there arises a contradictory demand to reduce the magnetic force of the motor magnet.
Therefore, an object of the present invention is to provide a rotary electric machine which is capable of utilizing reluctance torque when this rotary electric machine is used as a motor and thereby increasing the range of rotation speeds of the motor.
Another object of the present invention is to provide a rotary electric machine capable of preventing overcharge of the battery in its high-speed rotation and obviating the use of the voltage-reducing circuit when the rotary electric machine is used a starter-motor-cum-generator in vehicles, such as motorcycles.